Citric acid is produced commercially by fermentation of certain organic substrates. The most common substrate for such fermentation are carbohydrates such as dextrose and sucrose, but it is also possible to use straight-chain hydrocarbons. Citric acid fermentation produces a so-called fermentation broth from which citric acid values are recovered in form of citric acid or citrates.
In certain industrial operations a virtually pure substrate of the kind specified is used to which only the necessary nutrients have to be added. In such an operation the recovery of clean citric acid or citrate by solvent extraction is relatively easy.
In other operations impure substrates such as molasses are used for the fermentation. Such impure substrates contain high percentages of organics that are not carbohydrates as well as various electrolytes. In such operations the resulting fermentation broth contains a substantial amount of impurities and the recovery of clean citric acid or citrate by solvent extraction is more complicated and impractical in practice.
U.S. Pat. No. 4,275,234 describes a solvent extraction process for recovering citric acid from broths obtained from the fermentation of pure substrates. In the performance of this process a water-immiscible organic extractant is used which comprises at least one secondary or tertiary amine in which the total number of carbon atoms per molecule is at least 20, dissolved in a water-immiscible, organic, non-polar or polar solvent, and the operation involves extraction of the broth with such an extractant at a low temperature and back extraction of the extract with water at a higher temperature. While this process provides for high recoveries of citric acid, it entails high costs. One source of the cost is the need to cool large volumes of fermentation broth and of solvent for extractiOn and to reheat solvent and heat water for back-extraction.
A further source of costs of the process according to U.S. Pat. No. 4,275,234 is the need to dispose of the aqueous extraction residue (broth raffinate) which contains substantially all of the water of the extracted fermentation broth and various, mainly organic substances such as residual carbohydrates, amino compounds, etc. Such disposal can be effected by evaporation of the bulk of the water and using the remaining concentrate, e.g. as cattle feed. Such commercialisation does, however, not compensate for the significant evaporation cost.
Alternatively, the aqueous extraction residue obtained in the process of U.S. Pat. No. 4,275,234 may be subjected to biological effluent treatment, but this again is cost intensive regardless of whether it is done at the citric acid plant itself or shipped to an outside sewage treatment system.
U.S. Pat. No. 4,334,095 describes a multi-stage total extraction process by which a citric: acid fermentation broth is extracted with a mixture of a water-immiscible amine and a water-immiscible organic acid dissolved in a suitable water-immiscible solvent, and the resulting extract is back-extracted with water. This process is, however, impractical and capital intensive.
It is the object of the present invention to provide an alternative simple process for the recovery of pure citric acid from a citric acid fermentation broth derived from fermentation of a pure substrate such as described above.